A more thorough explanation:

A rule of decision is a legal principle or guideline that serves as the basis for deciding a case. It can be a statute, a body of law, a prior court decision, or any other legal authority that provides guidance on how a particular issue should be resolved.

• Statutory law: A state law that sets the minimum age for purchasing alcohol is a rule of decision for cases involving underage drinking.
• Case law: A previous court decision that established a legal precedent for how to interpret a particular statute is a rule of decision for future cases involving that statute.
• Administrative law: Regulations issued by a government agency can serve as a rule of decision for cases involving that agency's actions or policies.

These examples illustrate how different sources of law can provide guidance for resolving legal disputes. By following established rules of decision, judges and other legal decision-makers can ensure consistency and predictability in the legal system.